The disparity between communication speed on an electronic device such as an integrated circuit or “chip” (on-chip bandwidth) and communication between two chips (off-chip bandwidth) is increasing to a point that the off-chip bandwidth becomes a bottleneck that limits the overall system performance. Factors leading to this increased disparity include continuous scaling of VLSI line-width and increasing on-chip clock speeds. Off-chip communication is expected to become more important with advances in high performance computing systems that are based on a massively parallel architecture.
To improve off-chip bandwidth, work has been done to increase the off-chip data rate using high-speed serial transceivers. However, issues such as topological limits, contact and parasitic RC limits, and power dissipation limits on driving low-impedance off-chip lines limit the off-chip bandwidth that can be achieved using high-speed serial transceivers.
A technique that has been suggested to improve off-chip bandwidth involves the use of photonics on silicon. On-chip communication using photonics involves transmitting optical signals in a waveguide, which may have a sub-micrometer cross-sectional dimension. Communication off-chip involves transmitting optical signals in optical fiber, which may have a cross-sectional dimension of several micrometers, or more.
However, coupling optical signals between the on-chip waveguide and the off-chip optical fiber presents a significant challenge due to the mode mismatch between optical fiber and waveguides. A mode relates to a self-consistent electric field distribution of the optical signal. More particularly, the electric field distribution in question is the component that is perpendicular to the direction in which the optical signal propagates through an optical fiber or waveguide. A waveguide or an optical fiber may have one or more modes in which the electric field distribution of the optical signal is substantially self-consistent as the optical signal propagates (although the phase may change). Largely due to differences in cross-sectional dimensions of waveguides compared to optical fibers, any modes that may exist in a waveguide are typically very different is size from modes that typically exist in an optical fiber. As an example, an on-chip waveguide may have a sub-micrometer mode size, whereas, optical fiber typically has a mode size (or sizes) of about 6-9 micrometers.
Techniques have been devised to couple optical signals from optical fiber to a waveguide. For example, if a sub-micron sized waveguide is tapered at the end, an optical signal from an off-chip optical fiber can be coupled into the tapered end of the waveguide with little loss. This is a broadband approach that can accommodate a large range of wavelengths, but requires the optical fiber/waveguide coupling to be at the edge of the chip, and therefore does not allow a two-dimensional array of couplers to be formed on the surface of the chip. Thus, this technique does not allow wafer-scale optical testing of the waveguides and associated optical devices on the chip that are used to convert the optical signals to electrical signals.
In another technique, gratings have been used to couple surface-normal, or near surface-normal optical signals from optical fiber into sub-micron waveguides with low loss. However, because gratings are typically sensitive to a relatively narrow range of wavelengths, the wavelength bandwidth of the coupler is reduced. The grating is also sensitive to the polarization of the input optical signal, and hence extra coupling loss is unavoidable to achieve a polarization insensitive solution.
Therefore, improved techniques to allow high bandwidth communication between electronic devices, such as integrated circuit chips are desired.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.